The types of the method of producing a glass-particle-deposited body include an outside vapor deposition method (OVD method). In the OVD method, a burner is supplied with a glass-material gas, a combustible gas, a combustion-assisting gas, and an inert gas, glass particles are synthesized by a flame hydrolysis reaction or an oxidation reaction in a flame ejected from the burner, and the glass particles are deposited on a starting rod member (starting member) having superior corrosion and heat resistance to form a glass-particle-deposited body.
The types of the material for the starting member include glass, carbon, and alumina, for example. In addition, a starting member is also known that is composed of an eminently heat-resistive material having a heat- and corrosion-resistive coating. In the production of an optical fiber preform, a glass rod to become a part of the product, including the core, is sometimes used as the starting member.
In recent years, another OVD method has also been employed in which, in place of the glass-material gas, the glass particles themselves are fed to the burner so that the glass particles are ejected from the burner together with the flame to be deposited on the starting member. The two types of feeding methods may also be combined to carry out this process.
To improve the production speed of a glass-particle-deposited body in the OVD method, a method using a plurality of burners is known. This method is known as “a multiburner OVD method.” FIG. 5 is a conceptual diagram showing the multiburner OVD method. In FIG. 5, the numeral “1” indicates a starting member, and “2” indicates a glass-particle-deposited body being formed by the deposition of glass particles on the starting member 1. Burners A to C constitute a burner row. In FIG. 5, the movement of the burner row from above downward is referred to as the forward movement, and the movement from below upward as the backward movement. In the multiburner OVD method, the glass-particle-deposited body 2 is produced by ejecting a flame including glass particles from the burner toward the starting member 1 while the burner row and the starting member 1 are being moved reciprocatively relative to each other. To perform the relative reciprocating movement, either the burner or the starting member 1 may be moved.
The burner row starts its downward movement at a position where the position of the uppermost burner A in the burner row faces the uppermost position q0 in the glass-particle deposition range on the starting member 1. It reverses its moving direction at a position where the position of the lower-most burner C in the burner row faces the lowermost position q11 in the glass-particle deposition range and moves upward to return to the original position. The burner row repeats this cycle of movement. Consequently, the individual burners perform the relative reciprocating movement in their respective predetermined ranges on the glass-particle-deposited body 2. More specifically, the burner A moves between the positions q0 and q9, the burner B between the positions q1 and q10, and the burner C between the positions q2 and q11.
The column in the right-hand side of FIG. 5 shows the number of layers of the glass particles deposited on the starting member 1 when the burner row performs one reciprocation. When one burner passes once, one glass-particle-deposited layer is formed. Consequently, when one burner performs one reciprocation, two layers are formed. Therefore, the number of glass-particle-deposited layers produced by one reciprocation of the burner row becomes a constant value of six between the positions q2 and q9. The region in which the number of deposited layers becomes a constant value, such as described above, is referred to as a steady portion. In the end portions of the glass-particle-deposited body 2, which are the portions between the positions q0 and q2 and between the positions q9 and q11, the number of deposited layers decreases as the position approaches the end, and a tapered shape is formed. The region in which the number of deposited layers varies with the position, such as described above, is referred to as an unsteady portion. In the multiburner OVD method, the moving distance of the burner row relative to the starting member 1 is longer than the length of the steady portion.
In addition, “the divisionally synthesizing OVD method” is also known in which glass particles are deposited by using a plurality of burners placed at equally spaced intervals to constitute a burner row having a length corresponding to nearly the total length of the starting member. FIG. 1 is a conceptual diagram showing the divisionally synthesizing OVD method. In FIG. 1, the numeral “1” indicates a starting member, “2” a glass-particle-deposited body, and “3” burners (burners A to D). The burners A to D are placed at nearly equally spaced intervals to constitute a burner row 4. The glass-particle-deposited body 2 is produced through the following concurrent operations: the starting member 1 is rotated around its own center axis, the burner row 4 is moved reciprocatively along the length of the starting member 1, and each of the burners 3 ejects a flame 5 including glass particles 6 toward the starting member 1. In the divisionally synthesizing OVD method, the amplitude of the relative reciprocating movement between the starting member 1 and the burner row 4 is shortened, and the predetermined deposition segments on the starting member 1 are allocated to the individual burners 3.
FIG. 1 shows the case where the burner row 4 is moved relative to the starting member 1. Nevertheless, the starting member 1 may be moved relative to the burner row 4. Furthermore, the two members may also be moved relative to each other. In addition, FIG. 1 shows the case where the starting member 1 is held vertically, and accordingly the burner row 4 moves up or down. Nevertheless, the combination of the starting member 1 and the burner row 4 may be positioned at any orientation including the horizontal one to be moved reciprocatively.
In the divisionally synthesizing OVD method in early stages, the burner row 4 was simply moved reciprocatively over nearly the same distance as that of the burner interval. Thus, the deposition segments having a length corresponding to the burner interval on the glass-particle-deposited body 2 were allocated to the individual burners 3. In this case, the reversing position of the reciprocating movement of the burners 3 allows the accumulation of the influence of the increase in the deposition time due to the speed reduction of the burners 3 and the temperature rise at the deposition surface. As a result, variations in the diameter of the glass-particle-deposited body 2 tend to occur.
In order to decrease the diameter variation at the reversing position of the reciprocating movement in the divisionally synthesizing OVD method, “a zigzag method” has been proposed in the published Japanese patent applications Tokukaihei 3-228845 and Tokukaihei 4-260618. In “the zigzag method,” the range of the reciprocating movement of the burner row is predetermined to be an integral multiple of the burner interval. In addition, the moving distance in the reciprocating movement of the burner row is designed to be slightly different between the forward and backward movements. The reversing position of the reciprocating movement of the burner row moves successively from the position at the time of the beginning of the synthesizing and returns to the position at the time of the beginning of the synthesizing. This period is referred to as “one set.” The “set” is repeated twice or more. In the zigzag method, the reversing position of the reciprocating movement of the burner row is distributed over the starting member. Therefore, the factor causing the diameter variation at the reversing position can be distributed over the entire glass-particle-deposited body. As a result, the diameter variation in the glass-particle-deposited body is suppressed.
In addition, in the case where the reversing position of the reciprocating movement is fixed in the divisionally synthesizing OVD method, in order to decrease the diameter variation at the reversing position, “a condition-adjusting method” has been proposed in the published Japanese patent application Tokuhyou 2001-504426. In “the condition-adjusting method,” while the burner row is simply moved reciprocatively over the same distance as that of the burner interval, the deposition condition is adjusted at the reversing position to decrease the diameter variation. This adjustment of the deposition condition specific to the reversing position in the reciprocating movement of the burner row is referred to as “the condition adjustment at the reversing position” in order to distinguish it from the below-described alteration of the deposition condition associated with the growth of the glass-particle-deposited body. Hereinafter in the present specification, the term “the OVD method” includes the multiburner OVD method and the divisionally synthesizing OVD method (the zigzag method and the condition-adjusting method).
In the OVD method, alteration of the deposition condition other than “the condition alteration at the reversing position” is also performed. More specifically, as the diameter and surface area of the glass-particle-deposited body increase, the following alterations are performed:
(1) The distance between the starting member and the burner is widened to prevent the burner from making contact with the glass-particle-deposited body.
(2) The amount of the glass material or glass particles to be fed to the burner is increased to increase the synthesizing rate of the glass particles.
(3) The moving speed of the burner, the amount of the gas discharged from the reaction container for synthesizing the glass-particle-deposited body, and the flow rate of the combustible gas, combustion-assisting gas, and inert gas all to be fed to the burner are adjusted to suppress the cracking of the growing glass-particle-deposited body.
(4) The flow rate of the gas blown on the glass-particle-deposited body is adjusted to adjust the amount of the glass particles being deposited on the starting member.
Hereinafter, the foregoing items (1) to (4) are integrated into the expression “the alteration of the deposition condition associated with the growth of the glass-particle-deposited body,” which is shortened to “the alteration of the deposition condition.”
If any of the alteration of the deposition condition from (1) to (4) above is performed during the production process, in particular, in the cases where the relative position between the burner and the starting member is fixed at the time of the alteration of the deposition condition and where the alteration is performed significantly at a time, the alteration may cause variations in the diameter of the glass-particle-deposited body and variations in optical properties of the glass preform obtained by consolidating the glass-particle-deposited body.
In the multiburner OVD method, the alteration of the deposition condition is generally performed when the burner row is positioned at the reversing position of the reciprocating movement, i.e., by referring to FIG. 5, when the burner A is positioned at the position q0 (the burner B at the position q1 and the burner C at the position q2) and when the burner A is positioned at the position q9 (the burner B at the position q10 and the burner C at the position q11). This procedure enables the production of an excellent-quality glass-particle-deposited body 2 having small variations in the diameter at the portion other than the unsteady portion and bulk density. For example, the published Japanese patent application Tokukai 2000-44276 has stated that it is not desirable to alter the gas flow rate while the burner is positioned at the steady portion of the glass-particle-deposited body because the glass-particle-deposited body may include gas bubbles and other imperfections.
In this method, however, when the alteration of the deposition condition is performed, either the burner C is positioned at the position q2 or the burner A is positioned at the position q9 as a matter of course. This condition concentrates the influence of the alteration of the deposition condition at the boundary portions between the steady portion and unsteady portion of the glass-particle-deposited body 2 (the positions q2 and q9). As a result, the diameter variation and the like due to the alteration of the deposition condition tend to spread toward the steady portion from the positions q2 and q9. This tends to shorten the uniform-diameter portion of the glass-particle-deposited body 2, reducing the product yield and hampering the productivity.
In the divisionally synthesizing OVD method, all burners except the burners at the ends of the burner row are positioned at the steady portion at all times. Therefore, it is impossible to implement the method in which the alteration of the deposition condition is performed only when the burner is positioned at the unsteady portion. In particular, in the zigzag method, the alteration of the deposition condition is performed at the end of “the set.” In this case, the alteration of the deposition condition is performed invariably at the position where the burner row is positioned at which the synthesizing is started. Therefore, the disturbance in the deposition of glass particles due to the alteration of the deposition condition is concentrated at specific positions of the glass-particle-deposited body. Consequently, the specific positions suffer variations in the diameter and disturbances in the optical property in many cases. This undesirable circumstance has required to establish a method of producing a glass-particle-deposited body having a small diameter variation and excellent quality. In the above-described published Japanese patent application Tokukaihei 4-260618, no description has been given on the alteration of the deposition condition.
Patent literature 1: the Japanese patent application Laid open No. Hei 3-228845
Patent literature 2: the Japanese patent application Laid open No. Hei 4-260618
Patent literature 3: the Japanese patent application Laid open No. 2000-44276
Patent literature 4: the Japanese patent application Laid open No. 2001-504426